It has long been appreciated that gene expression can be regulated at the post-transcriptional level, defined to be the steps between transcription initiation and release of the nascent polypeptide from the ribosome. Antisense based approaches encompass a broad variety of techniques, but have in common an oligonucleotide that is designed to base pair with its complementary target mRNA, or more broadly to any RNA, leading to either degradation of the RNA or impaired function (e.g., impaired translation). Classical antisense approaches were designed to interfere with translation of the target mRNA or induce its degradation via Rnase H. Ribozyme-containing antisense molecules also can induce RNA degradation and have the advantage that they can be turned over (i.e., re-used) to cleave more RNA targets. RNAi-based approaches have proven more successful and involve using siRNA to target the mRNA to be degraded (see, e.g., Novina et al. (2004) Nature 430:161-4). However, some mRNAs are only modestly downregulated (2-fold) by RNAi and others are refractory.
The 3′ end processing (also called polyadenylation, poly(A) tail addition, or cleavage and polyadenylation) of nearly all eukaryotic pre-mRNA comprises two steps: (1) cleavage of the pre-mRNA followed by (2) the synthesis of a poly(A) tail at the 3′ end of the upstream cleavage product. 3′ end formation is essential to mRNA maturation and, in this sense, is as important as transcription initiation for producing a functional mRNA. 3′ end formation also functions to enhance transcription termination, transport of the mRNA from the nucleus, and mRNA translation and stability (Eckner et al. (1991) EMBO J., 10:3513-3522; Sachs et al. (1993) J. Biol. Chem., 268:22955-8). Defects in mRNA 3′ end formation can profoundly influence cell growth, development and function (see, e.g., Zhao et al. (1999) Microbiol. Mol. Biol. Rev., 63:405-445; Proudfoot et al. (2002). Cell 108:501-12).
Cleavage and polyadenylation requires two elements. The highly conserved AAUAAA sequence (also called the poly(A) signal or the hexanucleotide sequence) is found 10 to 30 nucleotides upstream of the cleavage site. This hexanucleotide is essential for both cleavage and polyadenylation and any point mutations (with the exception of AUUAAA) result in a large decrease in its activity (Proudfoot et al. (1976) Nature 263:211-4; Sheets et al. (1990) Nucl. Acids Res., 18:5799-805). However, recent bioinformatic studies have suggested that single-base variants or more-rarely double-base variants of AWUAAA (W=A or U) are allowed (Beaudoing et al. (2000) Gen. Res. 10:1001-1010; Tian et al. (2005) Nuc. Acids Res., 33:201-12).
The second element is a less-conserved U- or GU-rich region approximately 30 nucleotides downstream of the cleavage site and thus is called the downstream sequence element (DSE). Point mutations or small deletions do not greatly influence DSE's function. Nevertheless, the proximity of the DSE to the poly(A) site can affect the choice of the cleavage site and the efficiency of cleavage (Zhao et al. (1999). Microbiol. Mol. Biol. Rev., 63:405-445). The cleavage site itself (usually referred to as the pA site or poly(A) site) is selected mainly by the distance between the AAUAAA signal and the DSE (Chen et al. (1995). Nuc. Acids Res., 23:2614-2620). For most genes, cleavage happens after a CA dinucleotide.
In addition to the above signals, auxiliary sequences have also been found to have a positive or negative modulatory activity on 3′ end processing.
The cleavage/polyadenylation machinery is composed of multiple protein factors with some having multiple subunits. The endonucleolytic cleavage step involves Cleavage/Polyadenylation Specificity Factor (CPSF) binding to A(A/U)UAAA and Cleavage stimulatory Factor (CstF) binding the DSE. Other required factors include Cleavage Factors 1 and 2 (CF Im and CF IIm), RNA polymerase II (Pol II), Symplekin, and poly(A) polymerase (PAP), although the absolute requirement for PAP is still unclear. Once cleavage has occurred the downstream pre-mRNA fragment is rapidly degraded whereas the upstream fragment undergoes poly(A) tail addition that requires CPSF, PAP, and poly(A)-binding protein II (PAB II).
In principle, having the ability to switch on or off a gene's poly(A) site or sites is a way to directly control expression of that gene. There are natural examples where a gene's expression can be controlled by dialing up or down the poly(A) site. Perhaps the best understood example involves excess U1A protein negatively autoregulating its own synthesis by inhibiting polyadenylation of its own pre-mRNA. Without a poly(A) tail, the mRNA fails to leave the nucleus and is degraded leading to lower levels of U1A mRNA and U1A protein. The mechanism involves 2 molecules of U1A protein binding to a site just upstream of its own pre-mRNA's poly(A) site with the resulting (U1A)2-pre-mRNA complex inhibiting 3′-end processing of the U1A pre-mRNA by inhibiting the polyadenylation activity of PAP (Boelens et al. (1993) Cell 72:881-892; Gunderson et al. (1994) Cell 76:531-541; Gunderson et al. (1997) Genes Dev., 11:761-773). An illustrative example, albeit artificial, of “dialing” is found in Guan et al. (Mol. Cell. Biol. (2003) 23:3163-3172). Guan et al. demonstrate that endogenous U1A protein levels are dialed up or down by dialing up or down the activity of its poly(A) site through a stably-expressed epitope-tagged U1A protein that is under the control of a Tet-regulated promoter. The epitope-tagged U1A protein is not subject to autoregulation because its expression cassette lacks the autoregulatory 3′UTR element.
Another natural example of dialing a poly(A) site involves U1 snRNP binding to a “U1 site” just upstream of the poly(A) site of the bovine papillomavirus type 1 (BPV1) late gene pre-mRNA (Furth et al. (1994) Mol. Cell. Biol., 14: 5278-5289). The term “U1 site”, which stands for U1 snRNP binding site, is used so as to distinguish it from U1 snRNP's better known function in 5′ splice site (5′ss) binding during pre-mRNA splicing. U1 snRNP consists of 10 proteins in complex with a 164 nucleotide U1 snRNA that base pairs to the BPV1 U1 site via nucleotides 2-11 of U1 snRNA (see, e.g., Will et al. (1997) Curr. Opin. Cell Biol., 9:320-8), notably the same nucleotides 2-11 also basepair to the 5′ss sequence as part of the splicing mechanism. Subsequent to its discovery in BPV1, mechanistic studies demonstrated the U1-70K component of the U1 snRNP directly binds to and inhibits the polyadenylation activity of poly(A) polymerase (Gunderson, et al. (1998) Mol. Cell 1:255-264), the enzyme that adds the poly(A) tail. Additional studies in vivo that eliminated the U1-70K binding site confirmed U1-70K as the effector subunit that inhibits expression (Beckley et al. (2001) Mol. Cell Biol., 21:2815-25; Sajic et al. (2007) Nuc. Acids Res., 35:247-55). More recent studies have demonstrated U1 snRNP's polyA site inhibitory activity is part of a broader “surveillance” function that it has in quality control of RNA processing, in particular to suppress internal polyA sites that would otherwise lead to truncated mRNAs (Kaida et al., Nature (2010) 468:664-8; Berg et al., Cell (2012) 150:53-64).
The U1in gene silencing technologies use 5′-end-mutated U1 snRNA (see, e.g., U.S. Patent Application Publication Nos. 2003/0082149 and 2005/0043261). U1in stands for U1 snRNP inhibition of expression and refers to two recently developed gene silencing technologies that involve expression of a 5′-end-mutated U1 snRNA where nucleotides 2-11 of U1 snRNA are complementary to a 10 nucleotide sequence in the target gene's 3′ terminal exon. The 5′-end-mutated U1 snRNA is expressed from a U1 snRNA expression cassette containing promoter elements and a 3′ end formation signal from the U1 snRNA gene. The 5′-end-mutated U1 snRNA transcript assembles with the canonical U1 snRNP proteins into a 5′-end-mutated U1 snRNP that then binds to and inhibits polyadenylation of the targeted pre-mRNA. The 3 key features to make U1in silencing work are: (1) the U1 site on the target pre-mRNA and the 5′-end-mutated U1 snRNA must be perfectly complementary across all 10 basepairs, as a single base mismatch is sufficient to lose silencing (Liu et al. (2002) Nuc. Acids Res., 30:2329-39), (2) the U1 site must be in the 3′ terminal exon of the target pre-mRNA (Beckley et al. (2001) Mol. Cell Biol., 21:2815-25; Fortes et al. (2003) Proc. Natl. Acad. Sci., 100:8264-8269), and (3) the U1-70K binding site on the U1 snRNA must be intact. Although U1in has been successfully used in several instances, its development as a widely-used technology has been limited for a variety of reasons.
In view of the foregoing, it is clear that there is still a need for methods of regulating gene expression.